This illustration shows the nervous systems of the Alalcomenaeus fossil (left), a larval horseshoe crab (middle) and a scorpion (right). A new paper published in Nature reveals that the Alalcomenaeus is an ancestor to the group that includes both scorpions and horseshoe crabs.

Scientists have described the oldest complete central nervous system to ever be found: the brain of a 520-million-year-old fossil of an extinct marine animal.

The find, published this week in the scientific journal Nature, would be remarkable just as a simple superlative. But what makes the report all the more stunning is that this hundreds of millions years old brain looks, well, like a brain that is much more evolved than that of something hundreds of millions of years old.

“This was a very big surprise for us,” says Nicholas Strausfeld, a neuroscience professor at the University of Arizona and an author on the paper, “that such an ancient animal had such a sophisticated brain.”

The described fossil is a linchpin in scientists’ effort to piece together the evolutionary tree of the arthropods, the broad taxonomic group that includes modern insects, arachnids, and crustaceans and encompasses about four fifths of all known animal species.

It is a tree as complicated and nuanced as the veins in an insect’s wing, but one that, over the last year, is becoming ever clearer. Now, the new find, coupled with a similar find made last year, suggests that the two major branches that form the arthropod tree split from each other to develop their own complex brain systems as early as the early Cambrian period, or even earlier.

In other words, the brains in both groups of modern arthropods have obvious roots in the neural layout of organisms from half a billion years ago.

This week’s paper comes just one year after the same team published in Nature a description of another 520-million-year-old fossil with a complex brain. The fossil, Fuxianhuia protensa, had the primitive body plan expected of something dating to the Cambrian period, some 233 million years before the Triassic period when dinosaurs appeared. But it had a brain much more complex than the one that scientists had expected to find in something so old – a brain, in fact, like that of a mandibulate, the branch of the arthropod tree that includes modern shrimp and insects.

This meant that animals with mandibulate-like brains split from the arthropods to form a separate tree branch at least 520 million years ago. But what about the other, major group of arthropods, the chelicerates, which include modern spiders, ticks, horseshoe crabs, and scorpions? Might the forerunners to this big group of modern crawlers have also existed some 520 million years ago?

There was some evidence that they did. For years, scientists had been unsure where in the evolutionary tree to place an extinct group of marine animals known as the megacheirans, or “mega claw,” after the pair of scissor-like appendages at their heads. Scientists had suggested that these animals could be the ancestors to the chelicerates, since the elbow joint in the appendages at their head resembled the joint in the biting mouthparts of modern spiders and scorpions.

Still, that morphological evidence was not sufficient to prove that megacheirans were the chelicerates’ ancestors. That would require more substantial proof – it would require, in fact, a find as extraordinary as the Fuxianhuia protensa fossil: the scientists needed a megacheiran with a preserved brain.

As chance would have it, the fossil the team was hoping was found, plumbed from the Chengjiang formation in southwest China.

The 3-centimeter-long fossil, of an Alalcomenaeus, a member of the megacheiran group, contained what Strausfeld called “a beautifully preserved brain” – or, rather, three clusters of nerve cells, called ganglia, fused together as a brain and connected to other nerves running through the animal’s appendage-fringed torso.

The animal’s neural organization, mapped after combing a CT scan and a laser scan, left little doubt that the it was ancestral to the chelicerate group: Alalcomenaeus, more than half a billion years old, had a brain organized like that of modern chelicerate. That is, the animal had one optic sensor outside the brain mass and two sensors inside the brain mass. A mandibulate central nervous system, in contrast, has three optic sensors, all outside the brain mass.

“Our work has proved that, yes, it’s a chelicerate,” says Stausfeld. “There is no doubt about it.”

This fossil, coupled with last year’s fossil, means that arthropods must have diversified into the ancestors of both modern chelicerates and mandibulates much earlier than 520 million years ago. One estimate, based on plotting rates of genetic change over time, puts the split at about 545 million years ago, says Greg Edgecombe, a researcher at the London Natural History Museum and a co-author on the paper.

“Some of the geologically earliest arthropods had a nervous system organised in a way that is readily interpreted based on living arthropods,” says Dr. Edgecombe. “The basics of arthropod nervous systems evolved in the main burst of the Cambrian explosion.”

The team plans to return to China this May to look for a fossil with the ancestral central nervous system from which both the mandibulate and chelicerate brains derive. The hope, says Stausfeld, is that this ancestral group survived to live contiguously with the two more evolved groups.

“We hope that the ancestral type persisted,” says Strausfeld. “We have no idea if it did, but we’re looking.”